In single-use type applications, such as for example medical systems and instrumentation, discrete disposable sensors are required which can be implemented in a cost-effective manner. Typical flow sensors are not particularly well suited to such applications by virtue of the relatively high number of component parts, expensive materials and/or processing requirements, and high number of manufacturing-processing steps required to both produce the sensors and to integrate them into the instrumentation or apparatus of the application.
Existing discrete flow sensors utilize thermal microbridges, for example, such as those detailed in U.S. Pat. No. 4,651,564 to Johnson et al, which is assigned to Honeywell International Inc and is incorporated herein by reference. Such microbridge flow sensors include a flow sensor chip which has a thin film bridge structure thermally insulated from the chip substrate. A pair of temperature sensing resistive elements are arranged on the upper surface of the bridge either side of a heater element such that, when the bridge is immersed in the flow stream, the flow of the liquid or gas medium cools the temperature sensing element on the upstream side and promotes heat conduction from the heater element to thereby heat the temperature sensing element on the downstream side. The temperature differential between the upstream and downstream sensing elements, which increases with increasing flow speed, is converted into an output voltage by incorporating the sensing elements in a Wheatstone bridge circuit such that the flow speed of the gas or liquid can be detected by correlating the output voltage with the flow speed. When there is no fluid flow, there is no temperature differential because the upstream and downstream sensing elements are at similar temperatures.
Such thermal flow sensors suffer from a variety of drawbacks For example, the open nature of the microbridge structure can result in condensates from vapor being retained in the microbridge structure leading to uncontrolled changes in thermal response making the sensor measurements susceptible to error and instability. Additionally, the heater and sensing elements can heat the gas or liquid being measured. Furthermore, wires bonded to the heater and sensing elements retain particles suspended in the fluid and increase turbulence shifting flow response. Also, the wires are prone to damage in a high mass flux environment and during cleaning of the sensor.
Thermal flow sensors are high power consumption devices where are costly to replace and, as such, unsuitable for use as disposable flow sensors in medical and other applications. Calibration of each thermal flow sensor has to be conducted by trimming of calibration resistors which are mounted on a printed circuit board (PCB) connected to the flow sensor. Since each PCB is trimmed to calibrate a specific sensor, each sensor requires a dedicated PCB which must be disposed of when disposing of the associated flow sensor. Furthermore, manipulation of the medical apparatus by an operator is made particularly cumbersome by the fact that flow sensors and other associated devices have to be connected to the apparatus using a variety of wires and connectors.
There is a continuing need to provide improved disposable sensors which can be utilized in single use/disposable flow sensing applications more efficiently and cost effectively.
The embodiments disclosed herein therefore directly address the shortcomings of present flow sensors providing a pressure flow sensor system and a low cost disposable wireless pressure flow sensor for use therein.